1. Field of the Invention
No-Electronic Integrated Circuit (OEIC) is described such as may be used in Coherent Optical Switch (COS) systems, for example as described in co-pending application No. GB 9220592.1 entitled "Optic Switching". The OEIC greatly simplifies the coherent transceivers used to construct large port count optical switches based on coherent optical transmission. The OEIC also has the potential to be used in any coherent based optical transmission system requiring a transceiver function, not just coherent based switches.
2. Description of the Prior Art
In existing coherent based optical communication systems two lasers are required, one used as the coherent transmitter, the other being used as the local oscillator within the coherent receiver. The local oscillator is used as in a radio super-heterodyne receiver to tune into the coherent transmission channel. In multi-channel systems the local oscillator is used to select one of the many channels being simultaneously transmitted. Both the channel frequency and the inter-channel spacing of the transmitted channels need to be accurately controlled and stabilised.
The selectivity of coherent systems allows the formation of a switching function. A signal is transmitted on a given coherent channel. The `switching` function is performed by the destination port (coherent receiver) tuning into this source.
Some switch designs require a fully distributed switch architecture in which transmitter and receiver nodes can be located anywhere. In some architectures each transmitter and receiver pair are required to be located together. A realistic means of wavelength management of the transmitter laser is by use of an optical reference generated by a central management unit and fed to the transmitter via the same optical fibre link. This optical reference is tuned to the optical frequency of the transmitter it is controlling. A proportion of the transmitter signal power is used to act as a `local oscillator` and is mixed with the optical reference in the photodetectors to produce an electrical Intermediate Frequency (IF). Thus, using Automatic Frequency Control (AFC) techniques the transmit laser can be `locked` to this optical reference at a certain frequency `offset`.
The optical reference can either be present always or the transmitter laser be left to `free run`, being `nudged` back onto channel at set intervals. A further technique uses a direct detection link to transmit frequency correction information derived by a optical frequency monitor unit co-located with the central management unit. In this case the transmitter is `started-up` using the optical reference. Once "on channel", periodic frequency corrections are transmitted to the transmitter using a Wave Division Multiplexing (WDM) direct detection overlay link.